In the fabrication of composite structures, for example within the aerospace industry, structural members are often attached to a composite skin to provide reinforcement of the skin. Such structural members may include substantially elongated stiffening members often referred to as stringers or stiffeners. A typical example of the use of stiffeners within the aerospace industry is in the formation of an aircraft wing in which a composite stringer is attached to the inner surface of a wing cover, or skin. The stringers or stiffeners may be formed to exhibit various cross-sectional geometries but are typically formed to be T-section. Typically T-section stiffeners are produced from flat, uncured, pre-impregnated composite laminates that are formed into L-sections that are then compressed back-to-back producing the desired T-section. This produces a T-section stiffener that is easily attached to a flat composite skin. However, it is increasingly common practice to vary the thickness of the composite skin to locally vary the strength of the skin as desired. The variations in thickness produce ramped, or non-planar, sections across the composite skin to which the stringers are to be attached. To make the elongate stiffeners conform to the ramped skin surface the stiffeners must be deformed to the required profile after being formed into either the separate L-sections or the finished T-section. Since this involves deforming the stiffeners across a three dimensional section, rather than the two dimensional section of the original planar laminate, wrinkling often occurs in the T-section stiffener where a portion of the composite laminate is compressed. Such wrinkles provide localised stress points, thus reducing the overall strength of the stiffener and therefore requiring the stiffener to be made larger than would otherwise be necessary, and can also prevent L-sections from being compressed back-to-back to form the required T-section stiffener.
It would therefore be beneficial to be able to form such stiffeners without experiencing the problem of wrinkling.
EP 1 134 070 A refers to a process for manufacturing pre-cured parts of composite material with green-applied stiffeners, in which at least two parts manufactured in composite materials are structurally bonded. A base part or support is cured and a stiffener is in a green condition. The bonding of both parts is carried out by means of a structural adhesive sheet in such a way that the stiffener is compacted against the base part with adequate cross linking of the resin of its composite material, so strongly adhered to the skin of the base part.
WO 2005/105413 discloses an improved automated method and apparatus for forming composite structures by calendering two or more elements of a pre-impregnated composite material into a band of uncured composite material and then forming the uncured band of composite material into one or more formed, uncured, composite structures.